There has been growing dissatisfaction with the 12-volt battery currently used to power most automotive devices. Primarily there are two concerns: (a) the size of connected loads to the battery will grow in the future, e.g., with: (i) electric automotive air conditioning; (ii) using an electric motor to assist in acceleration and/or to actually launch a vehicle; (iii) high-power solid-state starting systems to enable “engine off” operation when stopped (i.e., to eliminate idling time); and (b) the difficulty of achieving high power, 1 HP or more at 12 V using semiconductors, due to heavy currents and the resulting high losses, especially when a semi-conductor bridge configuration is required (e.g., two active power devices in series, at currents of 75 A or more).
Other rather obvious problems also exist as desired power rises: (i) high cost of copper due to wire size at 75-100 Amperes; and (ii) the inefficiency of high powered alternators at low voltages, again due to series diode semiconductors in the power rectifiers they contain. In fact, automotive alternators over about 1.5-2.0 kW tend to be comprised of two alternators in parallel in one housing due to this inefficiency of high powered alternators, or water cooled (e.g., the 120 A or 150 A Bosch® alternators).
The Society of Automotive Engineers (“SAE”) Journal “Automotive Engineering” has had ongoing articles and even has described devices for the upcoming “42 Volt automotive electric system.” For example, see “Batteries for 42/14 Volt Automotive Electrical Systems” by Dell A. Crouch and Gary L. Ballard, published on Aug. 21, 2000.
The 42 V capacity (i.e., a 250% increase over current automotive batteries) was initially advocated by some to address the above problems. However, actual implementation of the idea has been “pushed back” year after year. This delay has happened for very good reasons, as some experienced individuals have looked at the drawbacks of such a system. The initial optimism has turned to confusion, although the advocates of 42 V do not see it that way.
Even with the 42 V approach, there are anticipated problems. For example, Applicant extrapolates that the battery for 42 V, if 42 is taken nominally as is 12 in 12 V batteries, requires 21 cells. This many cells in series poses a reliability problem as any slight difference in materials, or especially electrolyte quantity loading, or contamination will cause any one cell either to be better or worse than its neighbors in terms of exact Ah (i.e., battery capacity). This may lead to under- or over-charging of a cell compared to its neighbors, as the battery is cycled, leading to early failure of one cell out of the 21 cells.
This problem was severe when 12 V batteries first appeared in the mid 50's. The then-proven 6 V battery had three large cells, and was more forgiving of slight imbalances. However, for the first five years or so of 12 V battery use, “dead cells” were very common. As time passed, manufacturers achieved better matching, and this problem was essentially solved; but this was for six cells each about ½ the volume of the three-cell 6 V battery; a 21-cell battery will have about ⅛ the volume per cell of the 6 V type, so the sensitivity will be far higher to these problems. This history must not be dismissed.
It should be noted that 6 V, 12 V or 42 V batteries for an automobile will all hold about the same kWh, or energy total, and weigh about the same. The individual cell volume, however, decreases directly with voltage increase. The number of cells goes up, at the known rate of 2 V per cell, as voltage increases. That means more cells, smaller due to the same space constraints.
A 21-cell automotive battery will thus have small cells, and lots of them. It is expected those two negative trending facts will be multiplicative, and that the cell matching problem will reappear. Note that it takes only one cell to destroy the entire battery, even if the other 20 are in fine shape. There is a statistical aspect to this: the chances of a bad battery will be higher than just the linear aspect of more cells. Further, the cells are only about ¼ as large as a normal 12 V cell. Yet, they still will have to endure heavy currents more often than today, due to increased expectations such as the starting cycle at every stop; thus, the cells will fail more often.
Another problem with the multi-cell battery is that the charge systems in use, and envisioned for 42 V, can typically only make decisions by looking at the end voltage or terminal voltage. The multi-cell battery will stop a charge early if one or two cells cannot accept full charge, and thus are at a high voltage too early, even as the other cells are not yet charged. More elaborate charging systems (e.g., that monitor each cell) would help this, but they are prohibitively expensive, and make tapping each battery cell a necessity.
Last, for a given battery weight or physical size, more of the battery (with many cells) will be plastic cell separators and conductive cell connectors, rather than actual active material cell volume. One would expect the battery to be less efficient at storing energy per unit volume than a 12 V battery, yet cost far more to manufacture for the same kWh.
These problems are well known in the 48 V electric fork truck business, where the individual cells are much larger than the 12 V cell typical of automotive use. Most routine users of forklifts have encountered a bad battery. High voltage batteries comprised of many cells fail often.
Experience in the x-ray business highlights the same problems, with the batteries in portable x-ray units (e.g., the AMX® line by General Electric Company); three or four companies came into existence trying to solve battery problems with that x-ray unit, a 120 V array of batteries that constantly fail due to cell mismatch
Several potential hazards with unipolar 42 V batteries must be considered. For example, the 42 V battery provides energy well over that needed to sustain an “arc weld” type of electric arc. Any faults will tend to grow rapidly to major damage or fire, as fault currents will be hundreds if not thousands of amperes of DC at 42 V, which exceeds the values normally used for arc welding.
This will require large size, fast fuses; careful mechanical protection of cables; and added insulation. Yet the hazard still will exist, as when servicing the car, or in accidents.
Switching heavy DC currents at 40 V or more is problematic, since relays will need special materials, arc chambers or magnetic “blowouts”, when the current is higher than about 25 A. At less than 25 A, arcing of contacts in switches or relays is still a major problem, especially with inductive loads. For example, a commercial relay from OMRON Corporation for 200 A DC at these voltages costs $300.
Other commercial issues arise. For example, a 12 V sub-system still will be needed, as many automotive systems and devices are well developed and of reasonable cost at 12 V, such as power windows, radios, lighting. Accordingly, SAE expects initial applications of 42 V to also have a 12 V system, and possibly a 12 V battery as well as the 42 V battery. This leads to complex expensive topologies, involving DC/DC converters between the two systems to charge the 12 V systems. That will be an unworkable approach from a cost perspective, as its cost per automobile will be double (at least) any 12 V system.
The 42 V system is designed with one side grounded. Although the 42 V has been advocated primarily by solid-state engineers as being optimal for power semiconductors, a grounded system still will require an “H” bridge power stage configuration, or two semiconductors in series in each switching position, due to the grounded unipolar supply.
The optimal ideal automotive electrical system ought not to be driven only by one facet, the use of power semiconductors. It ought to be driven by a careful analysis of all the components and the topologies needed to employ them, as well as new hazards, and the battery issue.
Prior to semiconductors, circa 1912, engineers obtained several horsepower from 6 V batteries to start internal combustion engines, which demonstrates that voltage alone is not the limitation it appears to be.
U.S. Pat. No. 4,100,474 to Pfeffer et al. discloses a dual voltage which continues to use a common earth polarity for both voltages. Therefore, it does not allow bipolar operation. The same unipolar operation is true of U.S. Pat. No. 4,672,294 to Norton; Norton uses a series connection to achieve 24 V.
U.S. Pat. No. 5,874,822 to Navarro discloses an alternator construction with up to five windings to charge multiple batteries at the same time. Navarro does not address bipolar connection or bipolar charging; the intended alternator improvement is addressed toward very high outputs at any useable DC voltage, but particularly 12 and 24 V. Bipolar operation is not described.
U.S. Pat. No. 6,215,277 to Renehan discloses methods to charge two batteries of differing voltages of the same polarity from a single alternator. It does not disclose bipolar use.
U.S. Pat. No. 6,713,928 to Takizawa et al. discloses an alternator construction. That construction is specific to alternator mechanical design irrespective of electrical connections. Takizawa shows two sections in parallel to allow high output at one voltage.
U.S. Pat. No. 6,930,404 to Gale et al. (“Gale '404”) discloses the use of two 12 or 42 volt batteries which are charged in parallel by his switch SW1, SW2 and then switched to a series connection (i.e., 24 V) by a controller (i.e., element 41 in Gale's drawings). Gale's intent apparently is to provide 24 V or 84 V for a short period; for instance, while starting a car. The batteries return to 12 V or 42 V after the 24 V or 84 V operation is completed.
Gale describes many ways to charge and actively switch or use two batteries by control means, including various DC/DC converters, series parallel switching relays, or solid state devices, and boost converters.
Accordingly, it is a primary object of the present invention to greatly simplify the two battery concept described in Gale '404 by requiring no power switching, DC/DC converters or boost converters.
It is a more specific object to provide two batteries simply charged individually, permanently connected in bipolar and with a bipolar alternator, and used to power loads individually (with the batteries in series) without any intervening control means.
It is another object to provide a related method to power reversible DC motors.